1. Field of the Invention
The present invention relates to a cerium activated rare earth oxyhalide phosphor. Particularly, this invention relates to a cerium activated rare earth oxyhalide phosphor improved in afterglow characteristics.
2. Description of Prior Art
It has been heretofore known that a cerium activated rare earth oxyhalide phosphor having the following formula: EQU LnOX:xCe
in which Ln is at least one rare earth element selected from the group consisting of Y, La, Gd and Lu; X is at least one halogen selected from the group consisting of Cl, Br and I; and x is a number satisfying the condition of 0&lt;x.ltoreq.0.2, can be employed as a phosphor for a radiographic intensifying screen, since the phosphor gives an emission (spontaneous emission) in the blue light region with the maximum at a wavelength of approx. 380-400 nm when excited with a radiation such as X-rays. Recently, it has been discovered that said cerium activated rare earth oxyhalide phosphor emits light in the blue region when excited with an electromagnetic wave having a wavelength within the region of 450-900 nm after exposure to a radiation such as X-rays, that is, the phosphor gives stimulated emission. Because of the stimulability thereof, the cerium activated rare earth oxyhalide phosphor has been paid much attention and investigated as a phosphor for a radiation image storage panel employable in a radiation image recording and reproducing method utilizing a stimulable phosphor.
The radiation image recording and reproducing method utilizing a stimulable phosphor can be employed in place of the conventional radiography utilizing a combination of a radiographic film and an intensifying screen. The method involves steps of causing a stimulable phosphor to absorb a radiation having passed through an object or having radiated from an object; sequentially exciting (or scanning) the phosphor with an electromagnetic wave such as visible light or infrared rays (stimulating rays) to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at a considerably small dose, as compared with the conventional radiography. Accordingly, the radiation image recording and reproducing method is of great value, especially when the method is used for medical diagnosis.
The radiation image storage panel employed for the above-described method generally comprises a support and a stimulable phosphor layer provided on one surface of the support. However, if the phosphor layer is self-supporting, the support may be omitted. Further, a transparent film of a polymer material is generally provided on the free surface (surface not facing the support) of the phosphor layer to protect the phosphor layer from chemical deterioration or physical shock.
The stimulable phosphor emits light (gives stimulated emission) when excited with an electromagnetic wave (stimulating rays) such as visible light or infrared rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating the panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
The operation of reading out the radiation energy-stored image is generally carried out by the steps of scanning the panel with a laser beam (stimulable rays) to sequentially excite the stimulable phosphor so as to release the radiation energy stored therein as light emission and detecting the light by a photosensor.
In the last step of the read-out operation, the light which is continuously emitted by the stimulable phosphor of the radiation image storage panel after terminating the excitation with stimulating rays (namely, afterglow of stimulated emission) causes the decrease of S/N ratio of the resulting image. In more detail, the afterglow given by the phosphor particles other than the phosphor particles aimed to excite is detected as the light emitted by the aimed ones in the case that the phosphor gives afterglow in a relatively high ratio to the amount of the stimulated emission. As a result, the image provided by the radiation image storage panel comprising such a stimulable phosphor tends to deteriorate on the image quality (sharpness, density resolution, etc.).
The afterglow characteristics of the panel varies depending not only on the employed stimulable phosphor but also on the scanning speed of the stimulating rays. In more detail, if the scanning speed is slow enough, the afterglow affects the image quality only in a negligible small degree. However, the image processing is desired to be rapidly carried out, so the scanning speed needs to be high. In this case, the afterglow of the stimulable phosphor considerably lowers the image quality. Therefore, it is desired that the amount of afterglow of the stimulable phosphor employed for the radiation image storage panel be made as small as possible. In other words, it is desired that the stimulated emission cease as soon as the excitation with the stimulating rays terminates.
When a radiation image storage panel containing a stimulable phosphor is employed in radiography for medical diagnosis, it is also desired that the sensitivity of the panel to a radiation be made as high as possible to reduce the exposure dose for patient and to facilitate the procedure for converting the stimulated emission to electric signals. Accordingly, it is desired to make the luminance of stimulated emission of the phosphor employed for the panel as high as possible.
The cerium activated rare earth oxyhalide phosphor expressed by the above-described formula consists essentially of cerium as an activator and LnOX as a matrix crystal which has the PbFCl-type crystal structure and which is composed of rare earth element Ln, oxygen O and halogen X. The expression of LnOX in the above-described formula means that rare earth element Ln, oxygen O and halogen X together consist in a matrix crystal whose structure is the same as that of PbFCl crystal, and the expression does not mean that the atomic ratio of Ln, O and X is always 1:1:1 in the crystal.
Among the cerium activated rare earth oxyhalide phosphors expressed by the above-described formula, a phosphor of which ratio between Ln and X (X/Ln) satisfies the condition of 0.500&lt;X/Ln.ltoreq.0.998 by atomic ratio has a maximum peak of the stimulation spectrum located at .lambda. which is satisfying the condition of 550 nm&lt;.lambda.&lt;700 nm. The wavelength of the maximum peak of this phosphor (.lambda.) is longer than those of other phosphors and matches with a radiation wavelength of He-Ne laser, which is generally employed for a stimulating light source. Therefore, the phosphor can absorb the stimulating ray sufficiently and its luminance of stimulated emission is considerably high. With respect to the above-mentioned phosphor, a radiation image recording and reproducing method and a radiation image storage panel employing the phosphor, the inventors have obtained U.S. Pat. No. 5,003,183.
The cerium activated rare earth oxyhalide phosphor described in the specification of U.S. Pat. No. 5,003,183 exhibits high luminance, and the radiation image storage panel employing the phosphor has high sensitivity. However, the amount of afterglow of the phosphor is considerably large and the image provided by the panel employing the phosphor is lowered on the image quality when the scanning speed of the stimulating rays is high. Therefore it is desired to improve the afterglow characteristics of the above-mentioned stimulable phosphor.